Gain controlled optical fibre amplifier

ABSTRACT

In a gain-controlled erbium-doped optical amplifier, gain control is achieved by clamping the gain of a laser cavity to be equal to the overall cavity loss and by fixing the resonant wavelength of the amplifier to be at a first wavelength. When an optical signal to be amplified having a second wavelength different from the first wavelength passes through the amplifier the gain experienced by the signal depends entirely on the gain of the cavity, and not on the intensity of the signal. If the first wavelength is arranged to be at the peak of the sum of the absorption and emission cross sections of erbium, the amplifier exhibits minimum sensitivity to ambient changes in temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical amplifiers and, in particular, but notexclusively, to a rare earth doped fibre amplifier with improvedoperating characteristics.

2. Related Art

Rare earth doped optical fibre amplifiers are ideal as repeaters,pre-amplifiers or the like in optical communication systems. Thedesirable features of such amplifiers include low noise operation,polarisation insensitivity and low insertion loss. One particularadvantage derived from using rare earth dopants is that they typicallyexhibit sharp spectral features. In contrast, transition metal ion-dopedmedia exhibit very broad absorption and fluorescence features. Of therare earth dopants, erbium, when used as a dopant in silica fibre, haslasing properties in the desirable 1550 nm window for opticalcommunications, and similarly, praseodymium operates in the desirable1300 nm window. For convenience only, erbium and praseodymium dopedsilica fibres will be referred to in the following description, althoughit is emphasised that the description applies equally as well to otherrare earth dopant species and host configurations.

The relatively long fluorescence lifetime of the upper state of theamplifying transition in erbium, compared with for example semiconductorlaser transitions, has many important implications. One implication isthat electrical noise on the bias supply to a semiconductor pump laserused to pump an erbium doped fibre amplifier introduces a degree ofmodulation on the gain of the amplifier at low frequencies of pump noisecomponents below 100 KHz. Therefore, electrical bias supplies for pumplasers need to be noise free at these low frequencies at least. Anotherimplication is that the propagation of a signal through an erbium dopedfibre amplifier can cause modification of the population inversion andtherefore a modification of the amplifier gain for its own and otherwavelengths. This effect is particularly marked for pulses that aresufficiently intense to saturate the gain. Severe pulse shaping canoccur as a result of this process.

In multi-wavelength multiplexed transmission, for example wavelengthdivision multiplexed (WDM) systems, modulation of the amplifier gain byone of the multiplexed signals can result in low frequency crosstalkeffects on the other signals. These effects are particularlysignificant, where the transient effects associated with turningchannels off and on can seriously disrupt other wavelengths. This effectcan be eliminated if the amplifier's gain, and hence gain spectrum, iscontrolled independently of input signal level.

Known systems for implementing independent amplifier gain control useautomatic gain control (AGC) in the form of opto-electronic or alloptical feedback loops, where the all-optical option is more desirablein terms of reduced complexity and cost. Also, opto-electronic feedbackloops suffer with limited speed of response and potential degradation ofthe amplifier noise response.

One method of making an amplifier gain independent of input signal usingan all-optical feedback loop has been proposed in European PatentApplication 92300519.3. The method describes a semiconductor pumpederbium doped fibre amplifier (EDFA) which has coupled to it an opticalfeedback loop which couples the output of the amplifier to the input ofthe amplifier. A narrow bandwidth filter coupled to the feedback loopallows selected control wavelengths of the amplified spontaneousemission to pass from the output of the fibre amplifier to the input ofthe fibre amplifier. The feedback signal has a control wavelength whichis different from that of the pump and the wavelengths of the signals tobe amplified. The feedback control signal in effect locks the amplifierin a ring laser configuration. Thus, lasing conditions are controlled bythe wavelength of the feedback control signal and the attenuation in thefeedback loop, and not the input power of the pump or signalwavelengths. The ring laser configuration necessitates the use of awavelength selective coupler (WSC) and three 3 dB couplers oralternatively three WSCs, which, in terms of manufacture, is a complexarrangement.

An alternative arrangement is proposed in "Gain control in erbium-dopedfibre amplifiers by lasing at 1480 nm with photoinduced Bragg gratingswritten on fibre ends", Delevaque et al., Electronic Letters, June 10th,Vol 29, no. 12. The arrangement involves writing Bragg reflectors at acontrol wavelength at both fibre ends of an EDFA, there being nointrinsic loss mechanism for the signal except for the reflectors andfor coupling losses due to fusion splicing.

SUMMARY OF THE INVENTION

In accordance with a first aspect, the present invention provides anoptical amplifier comprising:

an optical cavity defined by first and second reflection means, theoptical cavity incorporating a gain medium comprising an optical fibrehost including a rare earth dopant species and being tuned for resonanceat a first wavelength;

means for coupling an optical signal into the cavity at a secondwavelength different from the first wavelength;

means for coupling the optical signal, after amplification, from thecavity; and

pump means for optically pumping the gain medium to provide a populationinversion,

characterised in that the optical cavity is arranged to provide anasymmetric laser flux distribution along its length, and in that one ofthe first or second wavelengths is substantially equal to the wavelengthof the peak of the sum of the absorption and emission cross-sections ofthe dopant species.

Emission and absorption characteristics of the rare earth elements havebeen investigated in some depth and it is known which of the elements(or ions thereof) exhibit potential as dopant species in silica based orfluoride based optical fibres (see reference [1]).

As will be described in more detail below, the applicants havedetermined that cavity sensitivity to ambient changes, for example intemperature, can be minimised, by placing the resonant wavelength of thecavity near to the peak of the sum of the absorption and emissioncross-sections of the dopant species.

For erbium, as the dopant species, the applicants have determined thatit is possible and desirable to place the resonant cavity wavelengthsubstantially at the peak of around 1530 nm, since the signal wavelengthcan be anywhere in between about 1500 nm and 1600 nm.

For praseodymium, the peak of the sum of the absorption and emissioncross-sections and the optimum signal transmission wavelength of around1300 nm substantially coincide, so it is not normally possible toposition the resonant wavelength at the peak in this case as it isnecessary for the signal wavelength and the resonant wavelength to bedifferent. However, even if the signal wavelength is at the peak, it ispossible to design the cavity to resonate at a wavelength very close tothe peak.

Advantageously, the use of erbium supports lasing and thus amplificationin the important 1550 nm window for silica fibre optical communications,and the use of praseodymium supports lasing and thus amplification inthe important 1300 nm window for silica fibre optical communications.Other dopant species and host arrangements produce gain in various otherwavelength ranges of interest. For example, neodymium has been seen toexhibit potential for operation in the 1300 nm window and thulium hasbeen seen to exhibit potential for operation in the 1500 nm window [1].

There are several configurations of input port, output port and pumparrangement which can achieve the desired effect. The input signal to beamplified or the amplified output signal can be either coupled into thecavity directly, or through the first or second reflector into thecavity. Similarly, the pump source can be coupled to the cavity directlyor through either the first or second reflectors. It is clear thatwhichever arrangement is chosen, suitable couplers and reflectors needto be incorporated.

Preferably, in an optical amplifier according to the present invention,the pump is combined with the input signal to be amplified by awavelength division multiplexer (WDM) and subsequently the combinedsignal is coupled into the optical cavity through the first reflector.

The lasing wavelength of the optical cavity is fixed by the reflectorswhich force lasing at the required wavelength. Typically the reflectorsare optical gratings written into the core of fibre either side of thefibre amplifier by known methods. One method is described in detail inKashyap et al., Electronic Letters, page 730-731, May 24, 1990, Vol 26,no. 11, the contents of which are incorporated herein by reference.Optical gratings can have highly wavelength specific reflectivity and,unlike mirrors, gratings can be easily and robustly incorporated intooptical fibres and optical fibre systems.

The pump means provides energy to optically pump the laser cavity. Forcontinuous, reliable pumping, the pump source can be a high powersemiconductor diode laser typically using an MQW structure, operating at1480 nm or 980 nm when erbium is the rare earth dopant. It is possibleto pump erbium ions with other wavelengths of pump, for example, 807 nm.However, the 807 nm pump can suffer with reduction of efficiency due toexcited state absorption. Of the 1480 nm and 980 nm pump options, 1480nm is preferred since at 1480 nm single mode transmission is possible instandard 1550 nm fibre. However, there would be few-moded operation at980 nm in standard 1550 nm fibre, which results in less efficient use ofpump power.

In a preferred embodiment of the present invention, a side-tap gratingis incorporated in the fibre between the first and second reflectors,close to the second reflector. The grating acts to couple a portion ofthe light at the lasing wavelength into the cladding of the fibre. Thus,wavelength specific attenuation is incorporated into the optical cavityand the first and second reflectors can both be highly reflecting at thelasing wavelength. The reflectors, being highly reflecting, also preventstray light at the lasing wavelength getting into the cavity fromoutside.

In a further embodiment, side tap gratings are added externally toeither side of the cavity. This is a useful improvement as even highlyreflective gratings in a fibre may be only about 95% reflective. Thefurther side tap gratings are able to attenuate the remaining light atthe laser wavelength, which would otherwise escape from the laser cavityinto the network fibre cladding.

Unlike the system proposed by Delevaque et al., the present amplifierhas control parameters optimised for operation in the 1550 nm window ofoptical communications for erbium or for operation in the 1300 nm windowfor praseodymium. A full analysis of the nature of one particularexample of an erbium doped fibre amplifier is presented, which enablesoptimum values for lasing wavelengths and losses for specific signalgains to be applied. The skilled person will appreciate that theanalysis, although based on an erbium doped AlGe:silica fibre amplifier,is applicable to other rare earth dopant species and hostconfigurations.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings in which:

FIG. 1 is a representation of an embodiment of the present amplifier;

FIG. 2 is a graph representing amplifier gain against input signal levelfor various pump powers;

FIG. 3 is a graph representing the transient response of the presentamplifier to an input signal being switched on;

FIG. 4 is a graph representing how relative gain of one example of anerbium doped fibre amplifier varies with lasing wavelength for differentvalues of inversion;

FIG. 5 is a graph representing change in gain for an input signalwavelength of 1550 nm against varying lasing wavelengths of operation;

FIG. 6 is a graph representing gain at an input signal wavelength of1550 nm against lasing wavelength for varying values of single passcavity loss;

FIG. 7 is a graph representing gain at an input signal wavelength of1550 nm against lasing wavelength when optimised to reduce the effect oflasing wavelength shifts;

FIG. 8 is an example of a replaceable fibre grating unit; and

FIG. 9 represents further embodiments of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 illustrates an embodiment of the present invention. Foramplification in the 1550 nm optical communication window, an erbiumdoped fibre amplifier 29 provides the optical gain medium for a lasercavity, the cavity being defined by a first optical grating 26 and asecond optical grating 28, the first and second optical gratings beinghighly reflective at the required lasing wavelength, and highlytransmitting at other wavelengths, particularly the pump and inputsignal wavelengths. In this embodiment they define the lasing wavelengthof the cavity at 1520 nm. A WDM 21 combines a pump at 1480 nm, which isinput at port 22, with a 1550 nm input signal, which is input at port20, to be amplified. The combined signal subsequently couples into theoptical cavity through the first optical grating 26. In gain controlledoperation, the gain of the laser cavity at the lasing wavelength equalsthe loss of the laser cavity and is neither affected by the data signallevel at a wavelength different from the lasing wavelength or the pumppower level. The amplified data signal at 1550 nm exits the amplifierthrough the second optical grating 28 and is output from an output port24.

In FIG. 1 the embodiment also includes isolator 23 which prevents strayreflections and amplified spontaneous emission (ASE) from the amplifiertravelling backwards and possibly interfering with previous amplifierstages.

The side-tap grating 25, in operation, acts to couple a portion of thelight at the lasing wavelength into the cladding of the optical fibre.This partial reflection provides the necessary loss for stable laseraction. The provision of a suitable level of loss means that bothoptical gratings 26 and 28 can be, and preferably are, high reflectorsat the lasing wavelength.

Highly reflective gratings 26 and 28 are placed around the erbium dopedfibre amplifier to provide optical feedback, to form a laser cavity.Under lasing conditions, the amplifier's gain at the lasing wavelengthis determined by the cavity losses. Assuming that the amplifying mediumis homogeneously broadened, which is a very good approximation forerbium in AlGe:silica fibre at room temperature, clamping the availablegain at one wavelength defines the gain at all other wavelengths withinthe gain spectrum. The gratings 26 and 28, which are written into thecore of the optical fibres either side of the erbium doped fibreamplifier, are used as feedback elements for three primary reasons.Firstly, because this enables the amplifier to be an all fibre device,which is advantageous in terms of avoiding alignment problems and costs.Secondly, the gratings can have a relatively narrow reflection spectrumto define the lasing wavelength very precisely, and hence the inversionof the optical amplifier is clamped at a precise level also. This is ofimportance because, even for the same cavity loss, different lasingwavelengths will give rise to different net inversions, and hencedifferent gains at other wavelengths. Thirdly, the level of out-of-bandreflection and loss from photosensitive gratings is extremely low, andthis is of great importance for the stable operation of the amplifier.

To ensure that the noise performance of the amplifier is degraded aslittle as possible, it is necessary to minimise the power of the controlwavelength at the input end of the amplifier laser cavity, so that thelevel of local inversion is not overly reduced at this point. This isachieved by making the control cavity asymmetric, such that thereflector near the input is close to unity, with the output reflector orthe side-tap grating and high reflector combination chosen to give theappropriate value of controlled gain. In a practical implementation, itmay also be necessary to include further side-tap gratings 27 and 30, orsome other source of loss at the lasing wavelength, outside of the lasercavity to further reject the residual laser power, to avoid downstreaminterference elsewhere in the system. The basic performance of acontrolled amplifier according to the present invention is shown in FIG.2 which shows how amplifier gain varies with input power for variouslevels of pump.

FIG. 2 shows a typical erbium doped silica fibre gain controlledamplifier characteristic. In this case, the amplifier consisted of 25 mof fibre, which was erbium in SiO₂ --Al₂ O₃ --GeO₂ --P₂ O₅ host glasswith a refractive index difference of 0.013, an LP_(ol) mode cut off of1.2 μm, a core diameter of 5.3 μm and an Er³⁺ concentration such thatthe peak absorption at 1532 nm was 6.1 dB/m at room temperature. It wasassumed that the amplifying medium was homogeneously broadened which isa very good first order approximation. The graph shows that thecontrolled signal gain level is 16 dB for a signal level up to about -10dBm, when the pump power was above 20 mW. Below 13 mW, the amplifier wasout of the controlled region because the net population inversionprovided by the pump was not sufficient to provide enough gain tomaintain lasing at the laser wavelength.

For a pump power of 47 mW it can be seen from the graph that the gainremains within 1 dB of that required (16 dB) up to about -2 dBm inputpower, with <0.2 dB variation over the majority of the range.

Without gain control, at this level of pump power (47 mW), the smallsignal gain (i.e. a gain which has negligible effect on the level ofinversion) of an uncontrolled amplifier would be around 30 dB or so.This indicates that the gain control scheme according to the presentinvention is remarkably effective in keeping the gain constant over awide range of input power levels.

There is also seen to be an advantage of relative insensitivity to pumppower variation within the limits of the controlled regions. From thegraph it can be seen that this is true for input signal levels less than-10 dBm, for the 20 mW to 47 mW curves.

Practically, useful controlled signal gains are likely to be betweenapproximately 10 dB and 25 dB gain at around 1550 nm. However control isequally possible at other gains. The limits on the laser wavelengths aredictated by whether there is enough gain available to appropriatelysaturate the device. This is discussed below for a particular exampleand it is shown that, to obtain 16 dB signal gain, for example, thelaser wavelength must be longer than about 1510 nm for the givenamplifier.

Another measure of the effectiveness of the gain control mechanism is tolook at the response of the amplifier to a transient in the inputsignal. FIG. 3 shows the response when the input data signal was simplyturned off and then on. In the graph there is shown the input signal forreference, together with several traces taken with different inputlevels from -10 dBm, to +2 dBm. The relative levels of the traces areadjusted for convenience in viewing. The controlled trace follows theinput very closely, while the others, which are outside the controlledregion, show varying degrees of overshoot. Without the control, the -10dBm signal would show considerable overshoot. It is this overshoot whichleads to transient saturation in uncontrolled fibre amplifiers.

In practice, it is important for a gain controlled amplifier accordingto the present invention to be optimised. Two of the parameters whichcan be optimised are cavity loss and laser wavelength. In assessing thepotential use of the gain controlled amplifier, it is also necessary toassess its sensitivity to environmental changes and constructionparameters. The present invention enables the optimisation of gaincontrolled lasers, particularly with regard to stability and lack ofsensitivity to ambient changes.

The gain of the amplifier in the controlled region is not dependent onpump power and signal power but is determined solely by the laser cavityparameters. Changes in the cavity loss (equivalent in steady state tothe gain at the laser wavelength) and the lasing wavelength, change thepopulation inversion in the doped fibre and hence the gain at otherwavelengths across the amplifying bandwidth. Changes in the temperatureof the gain medium also slightly affect both the laser gain and therelationship of the laser gain with the gain at other wavelengths.

Changes to the cavity loss may occur due to long-term degradation of thefibre or grating reflectors or, on shorter timescales, as a result oftemperature variations which may detune the two grating reflection peaksaway from one another. The change in gain, g_(s), at the signalwavelength caused by a change in the laser cavity loss γ is given by:##EQU1## where: γ is the single pass cavity loss (=g_(l), gain at laserwavelength),

N_(t) is the dopant ionic density,

σ_(al), σ_(as) are the absorption cross-sections at the laser and signalwavelengths,

σ_(el), σ_(es) =the emission cross-sections at the laser and signalwavelengths,

ν_(l) and ν_(s) =the laser and signal frequencies,

ν_(o) =the frequency for which the absorption and emissioncross-sections are equal,

h=Plank's constant,

K=Boltzman's constant,

T=temperature in Kelvin.

For gain stabilised operation in the amplifier according to theinvention, there must be sufficient gain at the laser cavity wavelengthfor lasing to occur. Once lasing, the saturation due to the laser fluxin the cavity forces the overall gain at the laser wavelength (two pass)to equal the total cavity loss (two pass). The cavity loss is chosen inaccordance with the cavity wavelength in such a way that the requiredgain at the signal wavelength is obtained. In the paper by Delevaque etal., the amplifier proposed has a laser wavelength which is in part ofthe spectrum where there is very little gain for quite a high populationinversion. Thus, it is apparent to the applicants that it is notpossible to reduce the cavity losses of this amplifier by very much, andeven if it were possible to reduce the losses to zero, the maximumpossible inversion would still be around 65% with very little gainsuppression. As a result it would not be possible to increase the gaincontrolled signal input by much without lasing stopping.

The gain of an amplifier according to the present invention isdetermined by the gain in operation at the laser wavelength as the gainmedium of the amplifier is essentially homogeneous. As long as there isa lasing flux, the gain at the laser wavelength is fixed equal to thelaser cavity loss. Power changes in the laser flux compensate for thesignal power level changes. If the signal power exceeds a criticallevel, it will reduce the inversion below the level necessary tomaintain laser operation. At this point, the gain at the laserwavelength falls below the level maintained by the laser cavity loss andlasing stops. For signal power levels greater than this critical level,the amplifier performance becomes that of an equivalent uncontrolleddevice.

Using measured absorption cross-section data, dg_(s) /dγ is plotted asshown in FIG. 5. The graph in FIG. 5 shows that for a gain controlledamplifier in which the laser wavelength is 1480 nm, such as thatproposed by Delevaque, a change in cavity loss of 0.1 dB would result ina change of 0.22 dB in the gain of a 1550 nm signal. The same change incavity loss for a laser wavelength of 1530 nm results in only a 0.055 dBchange in signal gain. It is clear from the graph that laser operationat 1520 nm (as in our example above) is not the optimum wavelength ofoperation, although it is clearly much better than 1480 nm, however, itis clear from the results presented what the optimum ranges of operationare.

It is clear from the foregoing that it is highly advantageous to operatethe laser as close to the peak of the sum of the emission and absorptioncross-sections as possible. As shown in FIG. 5, for erbium dopedAlGe:silica fibre, this is between 1.52 μm and 1.55 μm.

In terms of optimising laser wavelength, for a laser cavity with a gain,g_(l), the gain at the signal wavelength, g_(s), is given by ##EQU2##where N_(t) is the ionic density of the dopant.

Density of erbium ions in the silica fibre host can be calculated fromthe peak absorption of the fibre, its peak absorption cross-section, itslength, the overlap between the peak wavelength mode and the radialdistribution of the ions in the fibre core. The figure used in ourcalculations is 3×10²⁵ ions/m³.

FIG. 6 shows g_(s) plotted for laser cavity losses between 1 and 37 dB.The turning points in the curves give the laser wavelengths at which anychanges in laser wavelength result in minimum change in signal gain. Theexpected gains are plotted as a function of single pass cavity loss.From this graph, it is possible to compare the sensitivities ofamplifiers set up with similar gains but having different lasingwavelengths. For example, in the case of an amplifier operating at asignal gain of 28 dB, pumped at 980 nm and lasing at 1480 nm, the gainchanges by 0.7 dB for a 1 nm shift in laser wavelength. In comparison,an amplifier pumped at 1480 nm in which the lasing wavelength is 1531 nmchanges in signal gain by 0.25 dB for the same 1 nm shift in laserwavelength. A device operating at a lower signal gain of 16 dB, forexample, varies in signal gain by less than 0.1 dB for the 1 nm laserwavelength shift.

From the differential of Equation 2, it is possible to extract the laserwavelength for which an amplifier operating at any particular gain willbe wavelength-insensitive. The differential can be expressed as ##EQU3##

Over a restricted range, it is possible to find one or more laserwavelengths for which, to first order, there is no change in signal gainwith laser frequency variation. Imposing dg_(s) /dγ_(l) =0 results in##EQU4## which is plotted in FIG. 7.

The frequency shift with temperature of the gratings used to form thelaser cavity is typically 0.01 nm/° C. around 1.5 μm. Therefore a 100°C. temperature shift would shift the wavelength by 1 nm. Qualitatively,from FIG. 6, it can be seen that this shift in lasing wavelength wouldhave a negligible effect on the signal gain if the laser wavelength werechosen using FIG. 7.

Changes in gain due to the temperature dependence of the doped ioncross-sections are small and approximately linear for typicaltemperature variations around room temperature. The effects of suchchanges can either be ignored or compensated for by a small shift in theoptimum laser wavelength.

FIG. 4 shows plots of the relative gain coefficients of a typical erbiumdoped AlGe:silica fibre. They are derived from relative absorption andemission cross-section data in turn normalised from a white lightabsorption measurement and a side-light (out of the side of the fibre)fully inverted fluorescence spectrum. Between the two extreme curves areplotted the relative gain coefficients for a variety of populationinversions.

The curves indicate the relationships between gain at differentwavelengths across the band for particular inversions, for example anamplifier operating at a population inversion of 70% and with a relativegain of approximately 0.2 at 1520 nm will have a gain of about 0.35 at1560 nm.

For a gain controlled amplifier according to the present invention, thelasing wavelength and optical cavity loss determines the level ofinversion. If the cavity loss were for instance 0.4 (in the relativeunits in FIG. 4), then for a laser wavelength of 1532 nm the amplifierwould operate at an inversion of 72%, for 1520 nm the inversion would beabout 90%, but for wavelengths shorter than 1515 nm or longer than 1570nm, no lasing would occur because there is not enough gain in the systemat such wavelengths to support it.

For a cavity with no loss and a laser wavelength of 1510 nm, the minimuminversion that the amplifier can work at controllably is approximately60%. The shorter the laser wavelength, the higher this minimum inversionbecomes. For instance, if an inversion of 70% were desired, it would beimpossible to achieve this with a laser at wavelengths shorter than 1490nm.

The above behaviour is typical of three-level amplifier behaviour.Four-level gain systems, for example those incorporating praseodymiumare not fundamentally limited for zero loss cavities in this way.Because there is no lower laser state absorption, there is always somegain at the laser wavelength. This means that the laser wavelength canbe anywhere within the gain band.

The degradation in signal-to-noise ratio at a particular point in athree-level amplifier is determined by the population inversion of thegain medium at that point. This also applies to a four-level laser withparasitic loss mechanisms.

For an amplifier pumped directly into its upper laser level, the maximumpopulation inversion that can be achieved is governed by the wavelengthof the pump source, greater inversions being obtainable for shorterwavelength pump fluxes. The presence of a lasing flux at a wavelengthlonger than that of the pump serves to degrade the signal-to-noise ratioby pulling down the population inversion. It is important to minimisethis effect in the design of the gain controlled laser cavity.

The noise performance of an amplifier is dominated by conditions at itsinput end. By minimising the power level of the laser flux at theamplifier input end, a higher population inversion can be maintained atthis point with a consequent improved overall amplifier noiseperformance. This can be achieved by constructing the laser cavityasymmetrically with its predominant losses located towards the outputend of the fibre, resulting in low laser fluxes at the input of theamplifier and high at the output. In this way it is possible to achievehigh local inversion at the input end of the amplifier while maintaininga given net inversion across the cavity.

One way of achieving this asymmetry is by incorporating a side-tapgrating near the output end of the optical cavity which couples light ata specific laser wavelength out of the fibre core to be dispersed in thefibre cladding. Side-tap gratings may be formed, for example, using themethod described in Kashyap et al. referenced above. In the reference,an optical reflection grating is written into a fibre portion bysensitizing the fibre portion to UV light and impressing an interferencepattern into the body of the fibre portion using UV light. If the fibreportion is arranged in normal orientation to the interference pattern, astandard optical reflection grating is formed. If the fibre portion isarranged at an angle to the interference pattern, an angled, orside-tap, optical reflection grating is formed.

The side-tap grating at the laser wavelength can be inserted into thelaser cavity between the erbium doped amplifier and the second opticalgrating to determine the cavity loss. In this way, it is possible forthe first and second fibre cavity gratings to be two highly-reflectinggratings at the lasing wavelength. The advantage of encompassing thelaser within highly-reflecting gratings is that the laser cavity is lesssensitive to stray light at the laser wavelength that may be propagatingin the surrounding network due, for example, to ASE from other devicesor to external reflections. A further advantage is to reduce the effecton the external system from light at the laser wavelength in the lasercavity.

One of the prime advantages of a gain controlled amplifier according tothe present invention is that its inversion is uniquely defined and asingle passive filter can be constructed to equalise the gain of theamplifier over the bandwidth of the amplifier.

With reference to the example of a grating configuration shown in FIG.8, if a high reflection grating 26 with a broader reflection bandwidthis used at the input amplifier end, then the laser wavelength isuniquely determined by the reflective wavelength of a narrowerreflection bandwidth grating 38 at the output end. A side-tap grating 39controls the cavity loss and a set of gain-flattening gratings 40associated with the output end grating 38 control the wavelengthspectrum of the signal output from the amplifier. The gain-flatteninggratings 40 comprise a plurality of side tap gratings which couplediffering amounts of light at differing wavelengths into the fibrecladding, to equalise the amplifier gain spectrum of output light.

It may be useful to be able to reconfigure the amplifier for use atother fixed gains. An effective way of achieving this would be toconstruct the amplifier so that a gain conversion can be achieved byreplacing a single component comprising all the elements of FIG. 8 in asingle replaceable unit, the amplifier and the replaceable componentincluding appropriate demountable connectors.

It would be possible to arrange the output grating to be only partiallyreflecting at the lasing wavelength. This would have the disadvantage,in practice, that a highly efficient attenuator operating at the lasingwavelength would need to be incorporated with the amplifier, outside ofthe laser cavity, to prevent light at the lasing wavelength escapingfrom the amplifier into the subsequent optical system.

By placing the first and second optical gratings at the start and end ofa group of amplifiers, it would be possible to stabilise them alltogether. Thus a long distance communication link, of many tens orhundreds of kilometers could incorporate a gain controlled amplifiersystem comprising a single input and filter assembly and a single outputand filter assembly and many separate lengths of amplifying fibre, theamplifying fibres being separated by the distance dictated by thenetwork loss therebetween. Additionally filters could be added in thechain if necessary. Such an arrangement might also find application inpassive optical networks (PON), e.g. of the type proposed for localdistribution networks. Such distribution systems could be of greatimportance in WDM systems.

Generally, any modelling of any fibre amplifier to obtain specificinformation about actual expected gains requires quite detailedknowledge of fibre parameters. This includes the cross-section data forall the wavelengths of interest (pump and signal at least), rare earthion doping concentration, doping distribution in the fibre core, pumpand signal mode distribution in the fibre core and fibre length. Thisinformation combined allows a graph to be plotted with curves for therelative gain coefficients (as in FIG. 4), knowing what the scalingfactor is to convert this into actual gain. For a conventional amplifierthis means for any operating condition, both the amplifier's gain andthe inversion that it is at for that gain level is known.

Following from this it is possible to design a gain controlled device.Firstly from the graph (FIG. 4) it is necessary to read off the range oflaser wavelengths that can be used to provide the desired signal gain.Knowing the gain and range of wavelengths, it is necessary then tooptimise the system with respect to cavity loss. The first stage of theoptimisation involves selecting a laser wavelength which is as close tothe wavelength where the sum of the absorption and emissioncross-sections are a maximum (FIG. 5). In practice, for all rare earthfibre systems, the absorption cross-section peak and the emissioncross-section peak are very close and so putting the laser wavelength ateither of these is a good approximation. Further to this, if theamplifier is likely to undergo significant temperature variations thenthe next stage of optimisation involves looking for the appropriateminimum in the signal gain (or equivalently overall inversion) versuslaser wavelength (FIG. 7). From FIG. 5 and FIG. 7 it can be seen thatsuch minima exist in the vicinity of the loss optimum and so overallstability can be assured. Furthermore, the fact that changes in thecross-sections with temperature are approximately linear withtemperature over typical operation temperature ranges implies that withan additional small wavelength shift, this effect too can be minimised.

A further use for a gain controlled amplifier according to the presentinvention is use of the amplifier specifically set up to be used insaturation. In a saturated amplifier the output power of the amplifiedsignal is relatively independent of its input value which implies thatthe gain reduces as the signal increases.

By adding gain control to a saturated amplifier the maximum gain thatthe amplifier can provide is limited. In normal operation the inversionis determined by the high signal flux and there is no lasing at thelaser cavity wavelength. If the input signal level drops, however, thegain control prevents the gain from rising to what could potentially bevery high levels in high power amplifiers. This prevents spikingbehaviour and avoids problems with unwanted laser oscillations startingup spontaneously at transmission wavelengths due to small reflectionsfrom splices, multiplexers and other network elements.

As already indicated, the details given for an erbium doped amplifierare exemplary. Other rare earth dopant species can be used for otherwavelength ranges of operation. Praseodymium is one example.

Unlike erbium, praseodymium has a four-level lasing system which is ableto support a population inversion anywhere within the 1300 nm transitionbandwidth. Thus, there is no limit to the lasing wavelength within the1300 nm transition. Also, the gain spectrum for praseodymium is verysymmetrical which means that the wavelength region around the peak ofthe sum of the absorption and emission cross-sections (which in the caseof praseodymium is equivalent to the peak of the emission cross-section)will typically be occupied by signals. Although the lasing wavelengthfor a praseodymium-doped amplifier can be anywhere in the range 1250 nmto 1350 nm, the optimum lasing wavelength is around 1275 nm to avoidpossible ground state absorption (GSA) effects which tend to occur atlonger wavelengths.

Although, it is not possible to place the lasing wavelength of apraseodymium-doped amplifier at the peak of the sum of the absorptionand emission cross-sections, the advantages in having the laserwavelength as close to the peak as possible to give better controlremain.

A suitable optical fibre host for a praseodymium-doped amplifier is afluoride fibre, for example a ZBLAN fibre or a ZHBLAYLiNP (for highnumerical aperture fibres) fibre, doped to between 500 to 2000 ppm (byweight). A suitable ZHBLAYLiNP composition [Zr:Hf:Ba:La:Y:Al:Na:Li:Pb]for the core (in mol %) is [15:0:19:3.7:3:2.3:0:14:7], and for thecladding (in mol %) is [8.7:39.3:19:2.5:2:4.5:24:0:0]. A suitable pumpsource for the fibre is a Nd:YLF laser driven at around 700 mW, whichprovides light at around 1047 nm, this being well within thepraseodymium pump wavelength band of 950 nm to 1070 nm.

FIGS. 9a to 9e illustrate further alternative configurations of thepresent invention, the advantages of which are self-evident. It shouldbe noted that broken lines in these Figures signify optionalarrangements.

The applicants have determined that generally some dopant species, forexample erbium, have a peak of the sum of their absorption and emissioncross-sections which does not coincide with a desirable signaltransmission window wavelength, and in such circumstances the resonancewavelength can be placed at the peak for optimum gain stabilisation.Other dopant species, for example praseodymium, have a peak of the sumof their absorption and emission cross-sections substantially coincidentwith a desirable signal transmission window. Thus, these dopant speciesrequire that the resonant wavelength of an amplifier be placed, not atthe peak but as near to the peak as possible.

The amount of separation between signal and resonance wavelengthsdepends to a large extent on the ability of the cavity of an amplifierto distinguish between the wavelengths, or the achievable wavelengthselectivity of the reflectors, wavelength selective filters or couplerswhich define the cavity resonance wavelength and signal input and outputmeans. Typically, wavelength selective elements, for example opticalgratings, can distinguish between wavelengths as close as 5 nm apart.Therefore, if one wavelength is placed at the peak, the other can beplaced as close as 5 nm away. However, wavelength selective elementshave been shown to be capable of distinguishing between wavelengths upto 1 nm apart.

Obviously, for WDM systems in which the signal may comprise a pluralityof wavelength components all having wavelengths different from that ofthe resonant wavelength, it would be the wavelength component closest tothat of the resonant wavelength to which the above restrictions wouldapply.

REFERENCES

1. Rare earth doped fluorozirconate glasses for fibre devices, S. T.Davey & P. W. France, BT Technology Journal, Vol 7, No 1, January 1989.

What is claimed is:
 1. An optical amplifier comprising:a resonantoptical cavity resonant at a first wavelength, the cavity incorporatinga gain medium comprising an optical fibre host including a rare earthdopant species; a first coupling arrangement to couple one or moreoptical signals to be amplified into an input of the optical cavity, theor each optical signal having a wavelength different from the firstwavelength; a second coupling arrangement to couple the opticalsignal(s), after amplification, from an output of the optical cavity;and an optical pump source optically coupled to the optical cavity tosupply optical pump energy to the optical cavity to form, in use, a netpopulation inversion in the rare earth doped fibre host, wherein theoptical cavity is arranged such that said net population inversion isnon-uniformly distributed across said cavity from a first localinversion level at the input of the cavity to a second local inversionlevel at the output thereof, said first local inversion level beinghigher than said second local inversion level.
 2. An optical amplifieras in claim 1 in which the population inversion distribution issignificantly asymmetric because the optical cavity includes, towardsthe output of the cavity, at least one side-tap grating which providesgreater loss at the first wavelength than at the signal wavelength(s).3. An optical amplifier as in claim 1 wherein the optical cavity isdefined by first and second optical gratings which have their highestreflectivity at the first wavelength.
 4. An optical amplifier as inclaim 1 wherein the first wavelength is substantially equal to thewavelength of the peak of the sum of the absorption and emission crosssections of the dopant species.
 5. An optical amplifier as in claim 1the amplifier including wavelength selective elements, wherein thedopant species comprises praseodymium in a fluoride host, and in use oneof the signal wavelengths is substantially equal to the wavelength ofthe peak of the sum of the absorption and emission cross sections of thedopant species, the first wavelength is between 1250 and 1350 nm and thesignal wavelengths are sufficiently distant from the first wavelength tobe distinguishable therefrom by the wavelength selective elements of theamplifier.
 6. An optical amplifier comprising:an optical cavity definedby first and second reflectors, the optical cavity incorporating a gainmedium comprising an optical fibre host including a rare earth dopantspecies and being tuned for resonance at a first wavelength; a firstcoupling arrangement to couple an optical signal into the cavity at asecond wavelength different from the first wavelength; a second couplingarrangement to couple the optical signal, after amplification, from thecavity; and an optical source optically coupled to the optical cavity tooptically pump the gain medium to provide a population inversion,wherein the optical cavity is arranged to provide an asymmetric laserflux distribution along its length, and in that the first wavelength issubstantially equal to the wavelength of the peak of the sum of theabsorption and emission cross-sections of the dopant species.
 7. Anoptical amplifier as in claim 6 wherein, in use, the populationinversion in the cavity decreases along the length of the cavity fromthe first reflector towards the second reflector.
 8. An opticalamplifier as in claim 6 comprising erbium as the dopant species, whereinthe first wavelength is in the range 1510 nm to 1560 nm inclusive.
 9. Anoptical amplifier as in claim 8 wherein the fibre host for the erbiumdopant is an Al/Ge silica fibre, and wherein the first wavelength isbetween 1.52 and 1.55 microns.
 10. An optical amplifier as in claim 8,in which the pump source provides light at a wavelength of substantially1480 nm.
 11. An optical amplifier as in claim 6 wherein the opticalfibre host comprises predominantly praseodymium-doped fluoride fibre.12. An optical amplifier as in claim 11 wherein the first wavelength isbetween 1250 nm and 1350 nm.
 13. An optical amplifier as in claim 12wherein the first wavelength is arranged to be at least 5 nm from saidpeak.
 14. An optical amplifier as in claim 6 in which the opticalamplifier includes at least one side tap grating which providessignificantly more loss at the first wavelength than at the secondwavelength.
 15. An optical amplifier as in claim 6 in which the secondreflector has its highest reflectivity at the first wavelength.
 16. Anoptical amplifier as in claim 1 further comprising a gain flatteningoptical grating arrangement.
 17. An optical amplifier according to claim6 further comprising a gain flattening optical grating arrangement. 18.An optical amplifier comprising:an optical cavity, the optical cavitybeing defined by first and second reflectors and being tuned forresonance at a first wavelength, the first reflector being a broadbandreflector reflective at said first wavelength and the second reflectorbeing a narrowband reflector reflective at said first wavelength; anoptical fibre gain medium incorporating a rare-earth dopant speciesbeing provided in an optical path of the optical cavity; an input portfor coupling an optical input signal at a second wavelength differentfrom the first wavelength into the optical cavity; an output port viawhich the amplified input signal exits said cavity; and an opticalsource to pump the gain medium to produce a net population inversiontherein which supports steady-state lasing, where in the optical cavityis arranged such that said net population inversion is distributedacross said cavity from a first local inversion level at the input portof the cavity to a second local inversion level at the output portthereof, said first level being higher than said second level, and inthat said first wavelength is arranged to be substantially at the peakof the sum of the absorption and emission cross-sections of the dopantspecies.
 19. An optical amplifier as in claim 18 wherein the output portincludes said second reflector, the second reflector having a lowerreflectivity at the second wavelength than at said first wavelength. 20.An optical amplifier as in claim 18 wherein the second reflector has areflectivity of at least 90% at the first wavelength.
 21. An opticalamplifier as in claim 18 wherein the amplifier includes an arrangementdownstream of said second reflector to provide a significanttransmission loss at the first wavelength without significanttransmission loss at the second wavelength.
 22. A method of amplifyingan optical signal using an optical fibre amplifier, the methodcomprising the steps of:pumping an optical fibre host which includes arare earth dopant species and which is incorporated into an opticalcavity which is tuned for resonance at a first wavelength, to produce apopulation inversion; coupling the optical signal at a second wavelengthinto the input end of the cavity; and coupling the amplified opticalsignal from the output end of the optical cavity, wherein the populationinversion is asymmetric and falls from a first level to a lower secondlevel from the input of the cavity towards the output of the cavity. 23.A method of providing gain stabilized amplification of wavelengthdivision multiplexed optical signals in a communications network whichcomprises:coupling the wavelength division multiplexed signals from anoptical fibre of the communications network into the input of a rareearth doped optical fibre amplifier which includes an optical cavitywhich is resonant at a first wavelength different from the wavelengthsof any of the optical signals; and coupling the amplified opticalsignals out of the output of the amplifier into a further optical fibreof the communications network, wherein the optical cavity in use has anasymmetric population inversion distribution along the cavity, thedistribution falling from a higher level at the amplifier input to alower level at the amplifier output.
 24. A method as in claim 22 whereinthe first wavelength is substantially equal to the wavelength of thepeak of the sum of the absorption and emission cross sections of thedopant species.
 25. A method as in claim 22 wherein the rare earthdopant species is praseodymium in a fluoride fibre host.
 26. A method asin claim 23 wherein the rare earth dopant is erbium.